Transistor as a switch

hey! friends
i have to construct a two-ways switch. Can i use a transistor in that case. i means i am constructing a R2R ladder DAC circuit. in that circuit, there is a switch in two ways- originally ground and connect to ref voltage when switch is on. Please suggest me, which device is the most suitable for that switch.

Staff: Mentor

hey! friends
i have to construct a two-ways switch. Can i use a transistor in that case. i means i am constructing a R2R ladder DAC circuit. in that circuit, there is a switch in two ways- originally ground and connect to ref voltage when switch is on. Please suggest me, which device is the most suitable for that switch.

Welcome to the PF.

For an R-2R ladder DAC, it is generally much better to use CMOS gates to drive the inputs. Why do you want to use discrete transtors to drive the inputs? It will take at least two transistors and resistors per input (pull up and pull down), versus 1 gate per input, and you can get up to 8 gates per tiny SOIC package...

I think he means using the transistor as the switch element itself, not to control the switch. MOSFETs would be the easiest to use as the switch element. If you use a PMOS and an NMOS in parallel the switch should have fairly low, uniform resistance across the voltage range. You can find some ICs that have them already connected as analog switches (check Analog Devices) or you could use discrete transistors.

Staff: Mentor

Yeah, I honestly don't get the dual requirements of a bidirectional switch (in which case your suggestion is good), versus him saying he wants to build an R-2R ladder DAC. I'm not familiar with any need for a bidirectional switch in a ladder DAC, just drive elements for the inputs to the ladder.

First of all thanks a lot to berkeman and davidrit for your suggestions. i'm not familiar with
the electronic devices but i've to construct the R-2R DAC. In theorectical circuit diagram there are switches for each bit according to the resolution of DAC. The switches are controlled by the digital input of the DAC ('1' on, '0' off). the operation of the switch is when "off" state it connects the resistor network to ground and when "on" it connects the resistor network to reference voltage. My first idea is to use a transistor as that switch, but it was not worked. so if u don't mind,please tell me which device would be the most suitable one. I've to construct 14 bit DAC and using the Pspice Schematics software. Thank you.

Staff: Mentor

First of all thanks a lot to berkeman and davidrit for your suggestions. i'm not familiar with
the electronic devices but i've to construct the R-2R DAC. In theorectical circuit diagram there are switches for each bit according to the resolution of DAC. The switches are controlled by the digital input of the DAC ('1' on, '0' off). the operation of the switch is when "off" state it connects the resistor network to ground and when "on" it connects the resistor network to reference voltage. My first idea is to use a transistor as that switch, but it was not worked. so if u don't mind,please tell me which device would be the most suitable one. I've to construct 14 bit DAC and using the Pspice Schematics software. Thank you.

Use CMOS gates to drive the inputs of the ladder high and low, as I've already suggested. That is how real-world ladder DACs are driven. As long as you don't load the CMOS gates too much, they drive their outputs rail-to-rail, so the accuracy of your ladder DAC is pretty good and linear.

Taking a step back, it sounds like SPICE may be new to you and the circuit is new. Trying to learn SPICE and a circuit at the same time makes it much harder to do either.

Have you drawn your schematic by hand and hand calculated expected signal levels? You really need to do that if you are just starting out. SPICE is useful for quickly calculating how well things should work but you need to get an understanding of the circuit at a simple level first.

After your hand schematic makes sense then take the time to build up simple sections of the circuit in SPICE and simulate them. Then connect them together. That way things like how to call the correct model file and setup the analysis statements can be worked out without worrying about a complex circuit at the same time.